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THE CRUSTACEAN INTEGUMENT: SETAE,
SETULES, AND OTHER ORNAMENTATION
Anders Garm and Les Watling
Abstract
The cuticle pl ays an impor tant role in ma ny aspects of c rustacea n biology, since it is t he interfac e to
the su rroundin g world. Thus, t he cuticle di splays many s tructu ral speci alizat ions all over t he body.
The struct ures considered here are setae, setules, denticles, and spines. We provide def initions for
them and discuss their functional morphology and development, with the main focus on setae.
We recognize seven types of setae based on their detailed external morphology: plumose, pap-
pose, composite, serrate, papposerrate, simple, and cuspidate. In support of the categorization of
these setae, each seems to cor relate with a specif ic functional outcome such as feeding, grooming,
and locomotion. Setae are also important sensory organs, and in crustaceans they are normally
bimodal chemo- and mechanoreceptors, but there are also indications of thermo-, osmo-, and
hygrosensitivity. Little can be learned about the sensory functions from the external morphology
of setae, but their ultrastructure seems to provide better cues. In particular, mechanoreceptors
displ ay struct ures related to transduction mec hanisms, with the scolopa le as a good example. Still,
too few data are available outside malacostracans to draw general conclusions for all crustaceans,
underlining the need for multidisciplinary and broad intertaxon studies. Less is known about the
functional morphology and development of setules and denticles in the general cuticle, but they
seem to be homologous with similar structures on the setae. Arthropods outside Crustacea also
have seta e in their cut icle, and many s hared featu res can be found . They are especia lly well st udied
in insects, where many correlations between structu re and funct ion have been shown.
INTRODUCTION TO THE STRUCTURES OF THE CRUSTACEAN CUTICLE:
DEFINITIONS/CLASSIFICATION
One of the defining characters of crustaceans as well as other arthropods is their external skel-
eton, the cuticle (see also chapter ). The cuticle plays a major role in most aspects of crustacean
Functional Morphology and Diversity. Ed ited by Les Wat ling and M arti n Thiel.
© Ox ford Unive rsity Pres s. Publi shed by Ox ford Univer sity Pres s.
168 Functional Morphology and Diversity
biology, and this has led to a vast number of structural and functional specializations. Many
of these specializations lie within the detailed surface structures, and they are the topic of this
chapter. First, we provide an overview of the diversity of these structures and their functions
and use t his to suggest a classif ication system. The main part of t he review then focuses in deta il
on the major group of cuticular specializations, the setae, since they are by far the most studied
and have the greatest fu nctional di versity and importa nce. We end by comparing with data from
other arthropod groups and listing suggestions for where future research in this field is most
needed and will be most f ruitful.
When observing crustaceans with the naked eye, many of the cuticular specializations are
visible in a large number of species (Fig. .A). Some body parts and especially the appendages
appear furry (Fig. .B), and the hairlike structures found in these areas are outgrowths of the
general cuticle, normally with a distinct articulation at the base, making them flexible (Figs.
.D, .). There is a general consensus that these structures are homologous within Crustacea
and are a lso probably homologous w ith simi lar str uctures i n other art hropods. Ma ny terms have
been used for these structures, such as setae, sensilla, bristles, or even “hairs.” For crustaceans,
the most often used term is setae , and it will therefore be used here. Even though setae are in
general considered homologous, it is difficult to decide which cuticular projections to include
in this term. A number of authors have addressed this problem and provided def initions of what
they considered setae. Thomas () was one of the first to do so, and he proposed that all
elongate outgrowths with distal pores were setae. His work was based on light microscopy, and
electron microscopy work has since shown his definition to be far too narrow. Fish () con-
sidered elongate outgrowths f illed with “cytoplasm” as setae, but this very broad definition will
include many other structures, such as spines (see below), and exclude setae w ith no cells in the
lumen. Some authors have used the size of the cuticular structures as a basis for classification.
This has led to such terms as microsetae (Jacques ) and microtrichs (Cuadras , Steele
and Steele , ), but we do not approve of this approach. If a structure complies with our
given defin ition (see below), we will consider it a seta no mat ter the size, a nd we see no reason to
believe that they cannot be small. In fact, we believe that in small crustaceans, such as nauplius
larvae, there has been strong selection pressure for miniaturizing the setae.
An evolutionary perspective was ta ken by Watling (), who stressed the need for a def ini-
tion based on homologies. He suggested that the articulation with the general cuticle is such a
homology and used this structure to define setae from other cuticular outgrowths. This def ini-
tion has b een widely accep ted as it seems to hold tr ue for the vast major ity of setae, a nd the “stem
seta” probably also had such an articulation. When considering the diversity of present-day
crustaceans, though, Watling’s definition runs into some problems, which were first addressed
in an earlier review (Garm b).
Some of the articulated outgrowths have an ex ternal and internal morphology so similar to
long setu les found on some of the setae that t here are no stru ctural a rguments to consider them
as being different. They are commonly found on the mouthparts of decapods and peracarids
(Fig. .C), and we suggest that they should be included in the term setules (see below). The
other problem concerns a loss of the articulation between the setae and the general cuticle.
This has probably happened a number of times in several crustacean lineages to encompass
mechanical functions requiring a very sturdy seta (Garm and H ø eg , Garm a). Clear
examples of such loss are seen for the spinelike projection found on the basis of max illa of the
squat lobster Munida sarsi (Fig. .B). These unarticulated projections are innervated, have a
continuous lumen, and have a cellular arrangement very similar to other setae. Further, struc-
tures undoubtedly homologous with the spinelike projections (they are situated in the same
place and arranged in the same two parallel rows in other decapod species) are typical setae
with clear articulation (Garm b). The same situation is seen for unarticulated spinelike
Setae, Setules, and Other Ornamentation 169
AB
CD
EFG
Fig. 6.1.
Structures in the crustacean cuticle. (A) At the macroscopic level, many crustaceans, such as the hermit
crab Parapagurus sulcata , appear furry because of very heavy setation (picture courtesy of Dr. Jens T.
H ø eg). (B) Max illiped o f the hermit c rab Pagurus bernhardus di splaying he avy setat ion, especia lly on the
media l edges of the c oxa and basi s. Severa l types of s etae are pres ent. (C) Setu les from par agnath of P. ber n-
hardus are clearly art iculated w ith the general cuticle (inset). (D) Between the setae (S) on the mouthpart
of Panulirus argus , the cuticle is filled with teethlike structures (denticles). (E) Ultrastructure of setules
from the para gnath of Penaeus monodon shows that they a re made entirely of cuticle and lack a lumen and
inne rvation. (F) Ultra struct ure of setae show a round, hol low base fi lled wit h sheath cell s (ShC). Cu, seta l
cutic le. (G) Close-up o f the centra l part of the s etal lumen s howing t hat the semic ircul ar sheath ce lls (ShC)
encirc le the outer dendr itic segments (ODS) of a number of sensor y cells.
170 Functional Morphology and Diversity
Fig. 6.2.
Details of the external morphology of setae. (A) Spinelike setae from maxilla of Munida sarsi , with no
apparent articulation at the base (arrows). (B) Plumose setae displaying a supracuticular articulation
(arrows) wit h the general c uticle, ma king them very f lexible. (C) Most setae hav e an infracuticul ar artic u-
lation (ar rows) with the general c uticle, reduc ing their f lex ibility. (D) Setu les from a composi te seta show-
ing an art iculation (arrows) with the setal shaf t. (E) Some setae dis play unar ticulat ed teethl ike structu res
arranged parallel to the setal shaft. These denticles (De) are found in two rows on the distal half of the
setae of ten togeth er with sma ll setu les (S). (F) On some setae, t here is a gra duated tra nsforma tion betwe en
setu les (S) and dent icles (De). (G) Many setae d isplay a ter mina l pore (TP) at the tip, oft en associa ted with
smal l scalel ike setu les. (H) The tip of a se ta used for gr ooming the g ills . Such setae of ten have a spec ializ ed
tip. (I) A newly molt ed composite set a display ing a very di stinct a nnulus (rin ged) as a by-product f rom the
invagination du ring development.
Setae, Setules, and Other Ornamentation 171
setae on the dactylus of maxilliped of the shrimp Palaemon adspersus . The definition we will
follow here was put forward by Garm (b, ): “A seta is an elongate projection with a more
or less circular base and a continuous lumen. The lumen has a semicircular arrangement of
sheath cells basally.”
The available data on the ultrastructure of setae provide good support for the internal charac-
teristicsthe continuous lumen and the semicircular sheath cells (Fig. .F,G), also called envel-
oping cells (Alexander et al. , Hallberg et al. , Crouau , Paffenh ö fer and Loyd ).
That the sheath cells seem to be a unifying character for setae indicates that they play important
functional roles. They are involved in setal development, and this complex process could possibly
provide a more detailed definit ion. The continuou s lumen is also f unctional ly significa nt since it is
closely connected with the sensory properties of setae. Both of these topics are discussed in detail
in later sections.
It is often problematic to use internal characters because categorization is normally based
on light or scanning electron microscopy. The round shape of the basal part of a seta is there-
fore an important cha racter, and it seems to be very consistent for setae found on a ll body parts
of many groups of crustaceans (see Garm b for review). Still, using this character alone
will not suffice since it w ill not separate unarticulated setae and spines dealt with below.
Besides setae, there are other surface structures of the cuticle that we will briefly consider.
One group has already been mentionedthe setules. As said above, this is a term widely used
for certain outgrowths on setae, but we believe them to be a general feature of the cuticle. They
are elongate structures, – µm long, often inserted into the cuticle in a socket, making
them f lexible (Fig. .C). They are flattened in cross section and made entirely of cuticle, so
they are never innervated and do not contain semicircular sheath cells basally (Fig. .E). Most
often they have a serrated edge distally. Such setules are commonly found in the general cuticle
throughout the Crustacea, especially on the mouth apparatus and in the foregut, but they have
typically been referred to as setae (Halcrow and Bousfield , Holmquist , Martin ,
Olesen ).
Another expression often used when describing the cuticle of crustacean is denticles . Like set-
ules, denticles are commonly found on setae but also in the general cuticle. This again stresses that
some of the structures generally considered special features of setae are in fact general cuticular
characteristics. Denticles are relatively small structures (normally < µm long) and, as the name
implies, more or less tooth shaped (Fig. .D). They are unarticulated, made entirely of cuticle,
and never innervated. There is some evidence that they are in fact evolutionarily related to setules
(Garm b).
A common cuticular outgrowth is the spine. This term should be used with care since it
can be very hard to tell a true spine from an unarticulated seta. We consider a spine to be an
unarticulated extension of the general cuticle. It is hollow, and the lumen is lined with normal
epithelial cells; no innervation is present unless the spine carries setae (see, e.g., Martin and
Cash- Clark , their fig. A,C). If ultras tructu ral data a re not available , then compari son with
closely related species should be used to verif y that they do not have setae in the same position.
While the other types of structures are probably homologies, we find it very likely that spines
have arisen several times and represent convergent evolution.
Scales, like spines, are cuticular outgrowths, but they are generally wider than long and are
not usually hollow (Klepal and Kastner ). Most often, scales follow one side of the outline
of the polygons often visible on crustacean cuticles when observed with the scanning electron
microscope.
While it was long known that the crustacean cuticle was often sculptured, the exact
details could not be seen until the invention of the scanning electron microscope. Some of
the main features are summarized by Meyer-Rochow (), Holdich (), and Halcrow
172 Functional Morphology and Diversity
and Bousfield (). A terminology of surface sculpturing was proposed by Harris ()
for insects, but it seems equally applicable to crustaceans. The basic unit of sculpturing
seems to be a more or less well-defined polygon, which Hinton () and Duncan ()
assert represents the surface mani festation of underlying epidermal cells (see also chap-
ter ). Scales, microspines, micropores, and a large variety of other structures can be found
within and along the boundaries of polygons (e.g., Klepal and Kastner , Halcrow and
Bousfield ). In many other crustaceans and insects, however, the polygon is obliter-
ated by cuticular secretions that form more elaborate sculpture (e.g., Hinton , Meyer-
R o c how ).
THE EXTERNAL STRUCTURE OF SETAE
As discussed in the preceding section, setae constitute the largest and most diverse group of
structures, which is also why providing a general definition is not a trivial task. This diversity
is seen between species, but sometimes a single species carries close to the full diversity of seta
types. Most of the setae are found on the appendages, and especially the mouthparts are heav-
ily ornamented with setae, and a single segment (= article) of, for example, a maxilliped can
display quite a number of setal types (Fig. .B). In the following we wi ll try to deconstruct this
diversit y and pinpoint some of the important st ructures that cause the diversity. Str uctures that
share some kind of simi larities can be a product of the evolut ionary history of setae and thereby
be considered homologies, and/or they can be products of shared functionality. As we discuss
further below, most of the simi larities of setae stem from sha red functions, and this will be used
to suggest a classification system.
First, it is important to recognize that all setae can be seen as having a more or less elongate
and round (at least at t he base) central par t, the sha ft, whic h may or may not have speci alizat ions,
including different ty pes of outgrowths. The length of the shaft varies from just a few microm-
eters to several m illimeters in large decapods . They are found on a ll body parts, including inter-
nally in the foregut (A ltner et al. , Johnston ), and serve many different functions. This
diversity of function has undoubtedly added to the wide range of external morphology. Some
setae are long and slender with no apparent specializations along the setal shaft (Fig. .F,G),
whereas others have many types of outgrowths, resembling feathers or pine trees (Fig. .A,B).
Stil l others are t horn shaped or bent a nd appear as hooks . Despite the div ersity, several s ubstruc-
tures can be recognized in many setae a nd can be used to group the setae into dif ferent types. If
a classification includes too many details, there is a high r isk that the designated setal types will
be highly spe cialized and appear only in a ver y limited nu mber of taxa . Here, we try to avoid t his
problem and consider only overall structural similarities found in most major crustacean taxa,
since this will have the broadest application and interest.
One of the prominent substructures concerns how the setae attach to the general cuticle.
Three types of attachments are seen: () an articulation in the form of a socket, which is drawn
into the general cuticle and gives the seta an infracuticular articulation (Fig. .C)this is by
far the most common type of attachment; () the socket is extended from the general cuticle,
giving the seta a supracuticular articulation (Fig. .B)this gives the seta great f lexibility and
is often seen in setae experiencing large drag forces; () no articulation is seen, and the general
cuticle has a direct transition into the cuticle of the seta (Fig. .A)as mentioned earlier, the
articulation is probably reduced to obtain sturdiness.
A nother feat ure concern ing the stu rdiness is t he length:w idth (L:W) ratio of the sha ft, where
the width is mea sured at the base of t he seta. The va st majority of setae are sl im, with a L:W rat io
of > (Fig. .), but some setae are more stout and robust, wit h a L:W ratio < (Fig. .G). Surely
there are setae w ith intermediate L:W ratios, but they seem to be rare.
Setae, Setules, and Other Ornamentation 173
An annu lus situa ted on the proxi mal hal f of the shaf t is found on most i f not all setae (Fig. .I)
(Garm b). The annulus seems to have no function as such but is a by-product of the onto-
geny, when the setae develop in an invaginated state (for more details, see “Setal Development
and Ontogeny,” below). The annulus is most easily detected in newly molted animals since it
often diminishes as the cuticle thickens, stretches, and wears down during the intermolt stage
(Garm b).
Fig. 6.3.
Setal types. (A) Pappose setae, with long setules scattered along the entire shaft, from the mandibular
palp of Cherax quadrocarinatus. (B) Plumose setae, with two straight rows of long setules, from maxil-
liped of Munida sarsi. (C) Composite setae, with small setules distally, from maxilliped of Panulirus
argus. (D) Serrate setae, with two very distinct row of denticles, from max illa of C. quadrocarinatus. (E)
Papposerrate setae, with denticles and small setules distally and long setules proximally, from maxilla
of M. sarsi. (F) Simple setae, with an almost complete lack of surface structures, from maxilliped of P.
argus. (G) Cuspidate setae, from maxilliped of P. arg us. Some cuspidate setae can have small setules on
the middle pa rt.
174 Functional Morphology and Diversity
The tip of the seta also varies (Fig. .F–H) but is always more or less pointed. In cases of
a seta with outgrowths, the latter often form the very tip. A specialization of the tip is seen in
peracarids, where some setae have a bifurcate tip (Fish , Brandt ). Interestingly, the
thin “additional” tip holds all the sensory cells (Brandt ), and it might be a way of sepa-
rating mechanical and sensory functions. Another functionally interesting feature of the tip
is the presence or absence of a pore. The pore-bearing setae may have the pore in one of two
different positions: terminal or subterminal. Most common is a terminal pore situated at the
very tip of the seta, often bending to make the pore point to the side of the seta (Fig. .G).
Less common is a subterminal pore situated on the side of the seta a little proximal to the
tip. In at least some cases, the pore is associated with chemoreception (see later section for
detailed discussion).
The main substructures causing the diversity of setae are the presence or absence of out-
growths on the setal shaft and, when present, their detailed structure and arrangement. We
divide the outgrowths into two types: denticles and setules. Denticles as defined above are
rather small (normally < µm long), flat, and pointed outgrowths with smooth edges and no
articulation w ith the setal shaft. They are solid cuticle; that is, they lack a lumen. On the setae
they occur in two parallel rows, are always oriented with their broad axis parallel to the setal
shaft, and point distally (Fig. .E). Denticles arranged in this way are found distal to the
annulus.
Setules have a wide size range (– µm long), but they share common features. They all
have an articulation with the setal shaft, although, especially in smaller setules, the articulation
is often weak. They are all flattened, with their broad axis perpendicular to the setal shaft at the
point of attachment. Setules taper distally and often have a serrate edge, with most of the minute
tooth-shaped extensions distally (Fig. .D). Like denticles, setules are made of solid cuticle.
Setules always point toward the tip of the seta, but the angle changes with size. Long setules may
be almost perpendicular with the setal shaft, whereas small setules often lie almost flat against
the shaft (often referred to as scales). Long setules can be found along the whole length of a seta,
whereas small setules are normally found only distal to the annulus. They can form straight rows,
but most often they appear to be randomly arranged on the shaft. Even though denticles and set-
ules are normally very distinct and easy to separate, there are intermediate forms. In some cases,
rows of outgrowths gradually changing from setules to denticles (from base to tip) can be found
on the same seta (Fig. .F).
TOWARD A GENERAL CLASSIFICATION SYSTEM OF CRUSTACEAN SETAE
Setae are very diverse both in function and in morphology, thus necessitating a general clas-
sif ication system that w ill enable carci nologists to compare results obtained from a broad range
of taxa within Crustacea and possibly also with other arthropods. Many studies have provided
a setal classification system, but unfortunately, they are often based on a single or a few closely
related species and make use of a large number of st ructural deta ils (e.g., Garm and H ø eg ,
Coelho a nd Rodrig ues ). While t hat approach provides good insight into the seta l diversity
of the species studied, it has little or no value when setae in general are considered. Here we
try to set the basis for a more general classification system that can be used for most (if not all)
crustacean taxa.
We suggest that the vast majority of crustacean setae can be subdivided into seven catego-
ries or types based on the externa l morphological characters l isted earlier and, to some extent,
their mechanical functions: plumose, pappose, composite, serrate, papposerrate, simple, and
cuspidate (see also Table .). Note that the term serrulate used in earlier work (Watling ,
Setae, Setules, and Other Ornamentation 175
Garm b) has been changed to composite . The reason is that serrulate , meaning “small ser-
rate,” indicates the presence of denticles, which is never the case in these setae and why we
found it inappropriate. The reason for excluding the internal structures and thereby almost
all aspects of their sensory functions from this morphological description are that () we find
it unrealistic to expect that investigators in general can include these data since the quality of
the material (e.g., ethanol-fixed museum specimens) and time often do not allow this, and ()
at present there are too few ultrastructural data at hand outside decapods to draw trustworthy
general conclusions on the functional morphology and/or evolutionary histor y of the internal
structures. However, internal structure is very important for inferring function, as we illus-
trate below.
The morphological characters used to group the setae are not put forward as suggestions of
homologies, even though some of them might be, and thus, the seven types of setae are not to
be considered as separate evolutionary lines. It is in fact very likely that some of the types (e.g.,
the simple setae) are based on shared morphological characters that are convergently derived.
There is little doubt that strong selection pressures have acted on the external morphology of
setae, which comes from the many mechanical functions they serve. The outgrowths of setae
directly involved in food handling, for example, should be closely correlated with the nature of
the food items. Such a situat ion is bound to result i n convergences, and t his may to a large extent
impede a homolog y-based classification system.
Pappose setae. The shaft of pappose setae is long and slender, and they never display a pore.
They have long (– µm) setules with clear articulations scattered randomly along the
entire length of the shaft (Fig. .A). The setules have a serrated edge, with most teeth situated
distally, and they normally project with an angle between ° and ° to the setal shaft. The
socket of pappose setae is inf racuticular.
Plumose setae. Like pappose setae, plumose setae have long setules along the entire shaft,
but they are arranged in two strict rows on opposite sides of the shaft, giving them a feather-
like appearance (Fig. .B). The setules are never serrated, have a weak articulation with the
setal shaft, and are often situated in a groove. Plumose setae are the only setae that may have a
supracuticular articulation, and they never display a pore.
Composite setae. Composite setae are slim, wit h a naked proxi mal part, but have sma ll setules
(< µm long) distal to the annulus (Fig. .C). The setules can be arranged in rows or occur
Table 6.1. Summary of the characteristics of the seven types of setae.
Seta type Annulus Articulation Long
setules
Short
setules
Denticles Length:
width ratio
Pore
Pappose + Infra + +/– – > –
Plumose + Supra + – – > –
Composite + Infra – + – > Ter/–
Serrate + Infra – +/– + > Ter/
Sub/–
Papposerrate + In f ra + +/– + > Ter/–
Simple + Infra – – – > Ter/–
Cuspidate + Infra/absent – +/– – < Sub/–
Abbreviations: Infra, infracuticular articulation; Supra, supracuticular articulation; Ter, terminal; Sub, subterminal; +,
present; –, absent.
176 Functional Morphology and Diversity
randomly along t he shaft. T he setules are usually smaller in size toward t he tip and, for the most
part, point toward the tip of the seta with an angle of <°. The setules on composite setae can
be leaf shaped, digitate, or palmate. Composite setae may or may not have a terminal pore, and
the socket has an infracuticular articulation.
Serrate setae. Ser rate setae have a na ked proxima l half, but d istal to the a nnulus the y have t wo
rows of denticles point ing toward t he setal tip, with –° between t hem (Fig. . D). The dis-
tal ha lf may also have setules on the opposite side of the shaft from the denticles. When present,
the setules are small, have teeth along their edge, and lay almost flat against the shaft. Serrate
setae can have a terminal or a subterminal pore, and the articulation with the general cuticle is
infracut icular and of ten narrow.
Papposerrate setae. Like pappose and plumose setae, papposerrate setae are long and slender
(Fig. .E). On their proxi mal half to two-thirds (prox imal to the annulus), they have long, ran-
domly arranged setu les like pappose setae, but on t he distal pa rt they have two rows of dent icles
like serrate setae. In the area with the denticles, there may be additional small setules on the
opposite side of the denticles. The articulation of papposerrate setae with the general cuticle is
infracut icular and normal ly broad.
Simple setae. Simple setae are long and slender, and as the name implies, they completely
lack outgrowths on the setal shaft (Fig. .F). They have an annulus typically one-third of the
way up the shaft and a pointed tip, which may or may not have a terminal pore. They have an
infracuticular articulation.
Cuspidate setae. Cuspidate setae are very robust, with a L:W ratio < when width is meas-
ured at the base of the seta (Fig. .G). They have a broad base and taper gradually toward the
somewhat rounded tip. They may or may not have a subterminal pore, and in most cases they
have no outgrowths. They can have small setules in the midregion, often combined with a
weak curvature of the setal shaft with the outgrowths on the concave side. The distal part is
almost always naked but may be microtuberculate. In most cases they have an infracuticular
articulation with a very reduced membranous area, but sometimes the articulation is lacking
completely.
In our opinion, the vast majority of crustacean setae are easily put in one of the seven
described categories, but there is morphological variation within the groups, and this some-
times results in intermediate types of setae being described. Intermediate forms are occasion-
ally found bet ween the following types. () Pappose and plumose, where the prox imal part has
two distinct rows of setules, but on the distal part they are randomly spaced. () Pappose and
papposerrate, where the proximal part has long, randomly arranged setu les, but the distal par t
has short setules arranged in rows. () Pappose and composite, where the proximal part has a
few long setules, and the distal part has small or medium sized setules. () Composite and ser-
rate, where t he proximal most outgrowths are set ules, but they change gradual ly into denticles.
() Composite and simple, where the seta has a few small setules on the distal part. () Serrate
and cuspidate, where the outg rowths are denticle-like, but the d istal par t is naked, and the seta
has a L:W ratio between and . () Cuspidate and simple, where the seta has no outgrowths
and a L:W ratio between and .
It should again be noted that by far the most available data stem from malacostracans, espe-
cially decapods and peracarids. This can, of course, turn out to be fatal for the suggested clas-
sification system, but when going through the descriptions of nonmalacostracan crustaceans, it
seems that it will apply to those groups also (see Garm b for more details). The problem is
that none of the papers directly deal with setae, and therefore the pictures they present often do
not show the needed details. Further, since the authors did not make a survey of the seta, there
may be many other setae that are not shown. Hence, we might be able to say something about
which of these seven types are present but very little about whether additional types should be
named.
Setae, Setules, and Other Ornamentation 177
SETAL DEVELOPMENT AND ONTOGENY
The development of setae can be separated in two different events: () the development of a
new seta, and () the reconstruction of a seta during a molt. The first case is the more com-
plex and unfortunately only rarely studied. What little is known about formation of new setae
stems almost entirely from malacostracan olfactory setae, called aesthetascs . In decapods, the
aesthetascs are situated in discrete rows on the lateral f lagellum of antenna . The flagellum
is annulated, and each annulus has one or more rows of setae. With each molt, the distalmost
annuli with their aest hetasc are shed, and new annuli with aesthetascs are added prox imally on
the flagellum (Derby et al. , Ekerholm and Hallberg ). In the spiny lobster Panulirus
argus , in antenna a cell proliferation zone is found in the proximal part corresponding to the
area where the new aesthetascs will be situated (Steullet et al. a, Derby et al. ). Here
new olfac tory receptor neu rons (ORNs) are forme d conti nuously but w ith peaks in the prol ifer-
ation rate occurring in the premolt state. The OR Ns form in discrete clusters of about cells
that w ill in nervate a sing le aestheta sc (see fur ther below for more deta ils). In the se studies, l ittle
attention has been paid to the development of the rest of the seta, however. Earlier studies on
the development of the aest hetascs in peraca rids included obs ervations on t he role of the sheath
cells and cuticle formation (Guse ). Here it was also shown that already in the intermolt
stage the new ORNs are formed and that the sheath cells are also present to encircle them in a
cavity filled with receptor lymph. The number of sheath cells seems to vary, depending on the
size of the seta. Large and thick decapod setae can have as many as sheath cells (Fig. .F,G).
In the small setae of the isopod Idotea baltica , eight sheath cells were most commonly found
(Guse ). During the premolt, the sheath cells protect the OR Ns against the enzymes in the
exuvial fluid and start making the cuticle of the f uture seta. This process happens in an invagi-
nation of the epithelium, and the proximal part of the setal shaft is therefore also invaginated
(Fig. . A). The sheat h cells seem to wor k in pairs, a nd in the isopod ae sthetascs , the two inner-
most ones form the distal part of t he setal shaf t (Guse ). The next t wo pairs form t he middle
part, and the last pair forms the basis of the shaft and the socket. When the old cuticle is shed,
hydrostatic pressure everts t he seta with its ne w soft cuticle. The an nulus is left a s a “scar” i n the
new cuticle, indicating where the cuticle was folded during its formation.
The other type of setal development happens when the setae reform during a molt. Again,
litt le is known abou t the cellula r processes, and t he only ultr astruct ural dat a avai lable stem from
malacostracan aesthetascs (Guse , ) and for bimodal chemo- and mechanosensitive
composite setae of the shrimp Palaemon adspersus (Fig. .C,D). In the premolt stage, the epi-
thelium starts to pul l back from the cuticle, a nd in the isopod Idotea baltica , t he sheath cells also
pull back. T his means t hat the outer segments of the OR Ns are exposed to the exuvia l fluid and
are slowly digested, leaving the ORN insensitive for a period. The sheath cells pull back into an
invagination and start forming the cuticle of the new seta as described above. During this time,
the OR Ns rebuild their outer segments, wh ich is done by the time the old cut icle is shed and the
new seta is everted (Guse ). In other species such as the mysid Neomysis , not all the sheath
cells pull back; some stay in contact with the old cuticle and protect the outer segments during
most of the premolt s tage. The way the t issue pulls back f rom the old cuticle to for m the new one
is very standardized within species and is of ten used to stage the animal through the molt cycle
(e.g., Reaka , Longmuir , Hunter and Uglow ).
W hat is seen from the desc riptions above is that bot h new setae and molti ng setae are for med
in an invagination, leaving them with an annulus. This means that the annulus is a by-product
of the complex setal development and that it could be another important character defining a
seta. St ill, as st ated earlier, it ca n be hard to detect i n the intermolt , and there are ind ications th at
some setae are not developed in this way (Fig. .B; Watling ). This difference in ontogeny
suggests the question of whether these structures are true setae, which should be addressed
178 Functional Morphology and Diversity
with transmission electron microscopy. Even though the number of sheath cells seems to vary
greatly among different setae, their presence and similar arrangement leave the possibility that
the development might have many identical elements across crustacean taxa and setal types.
That sa id, far too l ittle data a re available f rom crusta ceans at present to ve rify t his. In i nsects, t he
development of their sensilla (believed to be homologous with crustacean setae) follows very
strict patterns when it comes to the cells involved (Keil ).
SETAL MORPHOLOGY AND SENSORY MODALITY
Mo st but not all ex amined s etae are sensor y organs of some k ind, and t hey are often ref erred to as
sensilla. There is experimental evidence for setae being mechano- and chemosensitive but also
indications of thermo-, osmo-, and hygrosensitivity. In most cases they appear to be bimodal
mechano- and chemoreceptors (see also chapter ). The number of sensor y cells that innervate
Fig. 6.4.
Seta l development. (A) Cr ustacea n setae form in a n invagi nation dur ing the premolt s tage. Af ter the molt,
the seta unfolds, due to hydrostatic pressure. The point of folding (arrowheads) becomes the annulus in
the new seta (see Fig. .). (B) In some cases the new setae form without an invagination, as shown here
from ma xil la of the amph ipod Uschakoviella echinophora . Aster isks ind icate new set ae inside th e exuvi um
(Ex). (C) Cross section of a composite seta during its formation in the premolt stage. Both the proximal
part of the cuticle (PC) and the distal part (DC) are lined with sheath cells (ShC). Outer dendritic seg-
ments (ODS) can be seen in the distal part. The lumen created by the invagination is partly filled with
extra-cellular matrix (ECM) closely encircling the setules (S). (D) Close-up of the tip of a forming seta
showi ng a thin c uticle (Cu) surround ing sheath c ells and outer d endritic se gments (ODS) with a dendr itic
sheath (DS) around the m.
Setae, Setules, and Other Ornamentation 179
crustacean setae can vary from one to several hundred. Despite this diversity, there are shared
features in the organization and ultrastructure of sensory setae. In this section, we provide an
overview of the sensory properties of setae and discuss whether they can be predicted by some
of the external or internal structures of setae.
One feature common to all sensory setae is having sensory cells with cell bodies arranged in
clusters outside of the seta they innervate and a single dendrite innervating the seta. The den-
drit e is composed of two pa rts, an i nner dendri tic segment (IDS) and a n outer dendritic s egment
(ODS). The IDS normally terminates at the setal base, but in some cases it continues into the
proximal part of the setal shaft. The ODS is a sensory cilium, and normally only a single ODS
protrudes from each IDS, but in the case of aesthetascs, the sensory cells often have two ODSs
(Gr ü nert and Ache ). In most cases, the ODS extends a long way up the lumen of the setal
shaft, but for some mechanoreceptor cells, the ODS terminates at the base of the seta, where it
contacts the cuticu lar part of the socket. Another shared featu re of sensory setae is the presence
of sheath cells encircling the sensory cells. Besides the described role during setal formation,
little is known about the function of sheath cells. They enclose the dendrites in a fluid-filled
cavity, and even though not much is known about this f luid and what the sheath cells add to its
content, the sheath is known to protect the ODS against stress factors from the external envi-
ronment, such as low salinity (Gleeson et al. ). The innermost sheath cell often has special
functions. In mecha noreceptors it contains a scolopale, which is probably a prerequisite for the
signal transduction (see below for more details), and in many but far from all setae, the distal
part of the innermost sheath cel l comprises the dendritic sheath, an extracellular structure that
encircles the ODS as it proceeds up t hrough the setal shaft.
Chemosensation has been associated with a broad spectrum of setal types through elec-
trophysiological and morphological studies. In fact, most of the examined setae appear to be
chemoreceptive. T hus, there seems to be little if a ny evidence for ex ternal seta l struct ures being
assoc iated wit h chemosensit ivit y. One cha racter t hat has been debat ed is the presenc e or absence
of a terminal or subterminal pore. There is some evidence that, at least for bimodal mechano-
and chemosensitive setae, a pore is needed for the ODS to contact the substratum (Fig. .C)
(Alexander , Garm et al. , Schmidt and Derby ). But in other studies the pore has
been shown to be blocked by electron-dense material and it has been suggested that the pore is
a holdover from molting, a so-called molting pore (Haug and Altner a). Further, the olfac-
tory aesthetascs almost always lack a terminal pore, showing that it is certainly not a necessity
for chemosensation in setae. I nstead, aest hetascs have a th in and porous cut icle, letting through
the small molecules they detect, such a s ami no acids, ammon ia, and adenosine monophosphate
(Derby et al. , Steullet and Derby , Derby , Steul let et al. b).
Interesti ngly, there m ight be ext ernal st ructu res that i ndicate the abs ence of chemosensat ion.
To our knowledge, setae with long setules along the entire shaft (plumose and pappose setae)
have never be en shown, inc luding from u ltrast ructu re, to be chemosensit ive. In some ca ses these
setae a re not innerv ated (Fig. .A), which sug gests that t hey have no sensory f unction . This can
be hard to tell, however, since mechanosensitive cells can be situated at a distance from a seta
and still detect displacement of the seta (Bender et al. ). In the few cases where they have
been shown to be sensory organs, plumose and pappose setae are mechanoreceptors detecting
waterborne vibrations (Vedel and Clarac , Wiese , Vedel ). This makes good sense
since the long setu les make the setae more sensitive to water movements, a nd crustacean mech-
anoreceptors have been shown to respond to even t he smallest setal displacements (Wiese ,
Fields et al. ). Arranging the setules in two rows, as for plumose setae, adds directionality
to the sensitivity. Long setules are by no means necessary for mechanoreception, though, and
mechanosensation has been either proven or at least indicated from ultrastructure for all major
types of setae (Tautz et al. , Derby , Garm et al. , Garm ). Regarding osmo-,
180 Functional Morphology and Diversity
Fig. 6.5.
Sensory properties of setae. (A) Cross section close to the base of a plumose seta showing several sheath
cells (ShC) but no sensory cells. S, setules. (B) Cross section of a serrate seta with both denticles (D) and
setules (S). The small lumen holds one outer dend ritic segment (ODS). (C) Section th rough the term inal
pore of a ser rate seta show ing the outer dend ritic seg ment protrud ing throu gh the pore. (D) Most bimod al
chemo- and mechanosensory setae have their outer dendritic segments (ODS) enclosed by a dendritic
sheath (DS) until close to the tip. (E) The ciliary region of chemoreceptors (top) and mechanoreceptors
(bottom). In mechanoreceptors, the a-strands of the microtubules are electron dense (arrowhead) and
have dy nein-like arms (arrow). (F) Almos t all mecha nosensory setae in c rustaceans a re of the scoloped ial
type. The scolopale (Sc) is made of single strands of microtubules (arrowheads) connected by a dense
matrix of accessor y proteins. (G) The inner dendritic segment (IDS) of mechanosensory cells displays a
large c ilia ry rootle t (CR), whic h has desmosom al connec tions (arrow heads) to the scolopa le (Sc). (H) The
outer dendritic segments (ODS) of most mechanosensor y cells have very den sely packed si ngle strand s of
microtubules, wh ich are thoug ht to enhance t heir sensitivit y to distor tions.
Setae, Setules, and Other Ornamentation 181
thermo-, and hygroreceptors, far too little data are available to evaluate whether these modali-
ties are associated with any particular external structures (Tazaki , Ache , Ziegler and
Altner , Garm et al. ). In conclusion, we find it highly questionable to address the sen-
sory functions of a seta from external characters alone even on the modality level.
From combined stu dies of sensor y physiology a nd interna l struct ure of setae, s ome struct ural
featu res, especia lly from t he sensory cel ls, can be use d to identify s ensory moda lity (Alt ner et al.
, , Schmidt and Gnatzy , Hal lberg et al. , , Derby et al. ). Interestingly,
information can be obtained at the histological level. The number of sensory cells can be deter-
mined in this way and in some cases can be used to ident ify chemosensitiv ity. From the existing
data, mechanosensation, and probably also osmo- and hygrosensation, is based on a maximum
of four sensory cells per seta (Mellon , Schmidt and Gnatzy , Ziegler and Altner ,
Garm et al. ). Chemosensation, on the other hand, is often based on many sensory cells,
with the aesthetascs of large decapods being the extreme, containing several hundred sensory
cells (Hallberg et al. , Schmidt and Derby , Hallberg and Skog ). The functional
explanation is probably that chemosensory setae need to detect a diversity of functionally rel-
evant chemical stimuli, which are often mixtures, and thus require a large number of receptor
molecules that are distributed nonhomogeneously across a population of sensory cells to iden-
tify the nature of those stimuli (Derby et al. , , Derby ). Since the functional unit
appears to be a single seta, each seta needs to contain this entire population of sensory cells
(Steullet et al. b). It is important to keep in mind, though, that many sensory cells are not
a necessity for chemosensation in setae. Especially in small setae, there is strong evidence that
chemosensation can be based on very few cells (Altner et al. , Elofsson and Hessler ,
Lagersson et al. ), but a small number of chemosensitive cells are also seen in some large
setae (Fig. .B) (Altner et al. ). How the low number of sensory cells in these setae influ-
ences source identification is unk nown.
On the ultrastructural level, there is better evidence for modality-specific structures. As
stated earlier, the ODS is a modified cilium and is the site of sensory transduction. For mech-
anoreceptors, it appears importa nt that the ODS is anchored, possibly ens uring that the sensory
cells do not just follow the movements of the setae but are distorted when a seta moves. This
anchorage is located in the dendrite just proximal to the ODS, where the scolopale is found in
the innermost sheath cell (Fig. .F,G). The scolopale is a very prominent structure made of
single strands of microtubules tied together by a large amount of associated protein (Fig. .F)
(Schmidt and Gnatzy ). The ciliary rootlets of the mechanosensory cells contact the scol-
opale through desmosomes, and this connection provides the anchoring (Fig. .G). Further
evidence for the role of the scolopale in mechanoreception comes from the fact that it is never
found in u nimodal chemorecept ive setae. Still, t here are some indications that putat ive osmore-
ceptors a re also connec ted to the scolopale, but these cells were bimoda l mechano- and osmore-
ceptors (Fig. .) (Garm et al. ).
W ithin t he sensory ce lls, the ci liar y struct ures seem a lso to provide in formation on mod ality.
The ciliary rootlet of the mechanoreceptors is often very large, perhaps to enhance their stabil-
ity. The transition zone from IDS to ODS contains the ciliary region of the sensory cilium and
displays the classical × arrangement of the microtubules in a ring. In mechanosensory, but
not chemosensory, cells the a-strand is electron dense and carries arms resembling the motor
protein dynein (Fig. .E). Further, the ciliary region of mechanoreceptors is several microm-
eters long, and it has been suggested that the stretch-sensitive ion-channels responsible for the
transduction from mechanical displacement to receptor potential are situated here (Crouau
). This is possibly tr ue for some mechanoreceptive cells, but in other cases the transduction
occurs farther up the cilium (Calvet , Praetorius et al. , Garm et al. , Garm and
H ø eg ). In insects, most of the mechanoreceptors are not associated with a scolopale, but
182 Functional Morphology and Diversity
instead, the dendrite ends in a tubular body composed of densely packed microtubules where
transduction is believed to take place (Keil ). Structurally similar sensory cells have been
found in association with setae of the terrestrial isopod Titanethes alba (Crouau ).
T here are als o indications t hat the dis tal part of t he sensory c ilium of mech anoreceptors ha s
special features. Their microtubules are often densely packed in most of the modified cilium
above the ciliary region, where the microtubules diverge in single strands (Fig. .H), perhaps
enhancing the sensitivity of the cell (French ). Interestingly, sensory cilia can be activated
by bending without disturbing the proximal part (Praetorius et al. ). This has also been
found in mouthpart setae of the spiny lobster Panulirus argus , and in accordance with this
Bend-sensitive cell
Bend-sensitive cell
A
B
C
DE
Stimulus
Stimulus
Stimulation on-set
Dis Dis
Bend
Dis
0 s
0 sec 2 sec 4 sec
DM
Se Cu
Cu
MC
S
CR
SD
LCR
Type 1
0.5 μm
Type 2 Type 3
ShC
SCR
WD
CR
DS
6 sec
2 s 4 s 6 s 8 s
Fig. 6.6.
Bimodal osmo- and mechanosensitive cells. (A) Electrophysiological recordings from mechanosensi-
tive cells in simple setae of Panulirus argus responding only to bending. (B) The same cell as in A did not
respond to displacements in the socket. (C) The bend-sensitive cells also responded to changes in osmo-
lar ity. Arrow heads indicate bend -sensitive cell; arrows ind icates additiona l smaller unit s in the record ing.
(D) The seta conta ining the bi modal osmo- a nd mechanosen sitive cell d isplays an i solated outer dend ritic
segment in the distal par t (arrowhead), which is suggested to be the bimodal cell. (E) Schematic draw ing
of the three types of sensory cells from simple setae in P. argus , identified by their ultrastructure. Type
is suggested as the bimodal cell. Abbrev iations: CR, ciliar y region; Cu, cuticle; DM, dense microtubules;
DS, dend ritic sheat h; LCR, la rge cilia ry rootle t; MC, membranou s cuticle; S, sc olopale; SCR , small c iliar y
rootle t; SD, strong des mosomal conn ection; Se Cu, s etal cuti cle; ShC, sheat h cell; W D, weak desmosom al
connection.
Setae, Setules, and Other Ornamentation 183
different way of stimulation, it was found that the cells have a short ciliary region, a weak con-
nection to the scolopale, and a small ciliary rootlet (Fig. .). The same cells also respond to
osmotic changes, and it is not known if any of the modifications of the structures associated
with mechanoreception are due to this bimodality.
Transduction in chemoreceptor cells is mediated through G-coupled receptors situ-
ated in the ODS (Couto et al. ). These cells seem to be in no need for anchorage, as they
are never connected to the scolopale, and their ciliary rootlet is small and often fragmented
(e.g., Snow ). Most li kely, as a fur ther consequenc e of their di fference in t ransduct ion, their
cil iary reg ion is short, of ten < µm, and the OD S has few loosely pac ked single st rands of micro -
tubules. The ODS of chemosensory cells continues to the very tip of the seta they innervate
(Fig. .B,C), but so far it has not been proven where the stimulation of the receptor molecules
takes place. The ultrastructural data indicate that there might be differences here. In bimodal
mechano- and chemosensor y setae, the ODSs a re wrapped in a dendritic sheat h up close to the
tip and sometimes also by projections of the sheath cells (Fig. .D) (Garm et al. , Garm
and H ø eg ). Even though nothing is known about the functionality of this arrangement,
it indicates that the ODSs are not in contact with the external environment except for the last
few micrometers. This is supported by the setal shaft having a thick and dense cuticle up to
the tip (Fig. .B). The situation is different in aesthetascs, which have no dendritic sheath,
a thin cuticle, and ODSs branching shortly after the ciliary region, resulting in a larger mem-
brane surface area (Gr ü nert and Ache ). Chemical stimuli can probably contact the ODS
along its entire length in aesthetascs (Blaustein et al. ), and these structural differences
may partly account for the reported threshold differences between the unimodal aesthetascs
and the bimodal mechano- and chemosensitive setae (Thompson and Ache , Voigt and
Atema , Garm et al. ).
A highly interesting and intriguing finding in sensory setae is the presence of bimodal
sensory cells. They can either be mechano- and chemosensors (Hatt ) or mechano- and
osmosensors (Tazaki , Garm et al. ). Nothing is known about the ultrastructural
organization of the former, but in the latter case there are indications that it is intermediate
between mechano- and chemosensors (Garm and H ø eg ). Bimodal cells have rarely been
identified i n cru staceans, but a ty pical experimental set up applies either mechan ical or chemi-
cal stimuli, and therefore, bimodal cells could easily have been missed because of lack of the
proper stimulus. Bimodal cells are also rare outside crustaceans but have been found in some
molluscs (Audesirk and Audesirk ).
Hygrosensitivity has never been proven by physiolog ical experiment s in crustaceans but has
been suggested from ultrastructure (Haug and Altner b, Ziegler and Altner ). Studies
on insect hygroreceptors resulted in five characters suggesting hygroreception: () the ODS
proceeds into the seta, () the setal shaft has no apparent pores, () the ODS borders the setal
wall distally in the seta, () the ODSs are organized symmetrically in the setal lumen, and ()
the seta l wall is layered. From our experience, at least the f irst, second, fif th, and to some extent
the third characters are found in most of the examined bimodal chemo- and mechanoreceptive
setae (e.g., Altner et al. , Crouau ).
Thermoreceptors are common in insect sensilla (Steinbrecht ), and several crustaceans
have been shown to respond to thermal stimulations and orient to a temperature gradient
(Barber ). The receptors responsible for these behaviors have not yet been identified, how-
ever, and might be other than setae.
To conclude, ultrastructural data suggest that crustacean sensory neurons have modal-
ity-specific structures at least for mechanoreceptors and chemoreceptors. Still, the largely
unknown bimodal cells and the possible presence of osmo-, hygro-, and thermoreceptors add a
question mark to what can be deduced about function of setae from their internal morpholog y.
184 Functional Morphology and Diversity
Until these modalit ies have been exa mined in greater deta il, no final conclusions can be drawn.
Furthermore, within each sensory modality, ultrastructure alone fails to reveal much about the
sensitivity of cells, such as what compounds and what concentrations stimulate a chemorecep-
tor cell.
SETAL MORPHOLOGY AND NONSENSORY FUNCTIONS
As shown in the preceding section, most setae are sensory organs, but setae also have other,
equally important functions. Especially important are mechanical functions during behav-
iors such as locomotion, digging, grooming, and feeding that mainly involve the setae on the
appendages. Combining macro-video recordings (Fig. .) with scanning electron microscopy
has provided evidence that the mechanical functions are largely correlated with the size, shape,
and locat ion of the setae and with the u ltrastructure of the cuticle. In the following, we descr ibe
some of the known mechanical f unctions of the setal t ypes and how they relate to t he setal mor-
phology (summarized in Table .).
The cuticular outgrowths of the setae seem to be the character that offers most insights
about thei r mechanica l function. Plumose setae wit h long fragile setules in t wo rows along their
entire length can be mechanoreceptors, but when situated on the pereopods or lateral mouth
appendages, they often serve as surface extensions used for water pumping or swimming (e.g.,
Kohlhage and Yager ). The two rows of long setules along the entire length of t he setal shaft
operat ing at low Rey nolds numbers ens ure a large su rface and t hereby a large d rag. They a re also
flexible, which in the case of the water pump on the maxillipeds of decapods is enhanced by an
annulated setal shaft (Garm and H ø eg ). Further, they are often arranged on the append-
ages such that they expand during the power stroke and collapse during the recovery stroke
(Burrows ). Pappose setae appear somewhat similar to plumose setae, but their setules are
randomly arranged along the shaft. Only in a few cases have their functions been studied, but
they have been shown to be involved in feeding in a number of decapods. In filter feeding deca-
pods such as gal atheids and thalassin ideans, they a re found in high densities on the ma xill ipeds,
where they work as filters hold ing back small suspended partic les (Nicol , Nickel l et al. ,
Stamhuis et al. ). In other crustaceans they are situated on the lateral mouthparts, perform
gentle prey handling, and in general create a setal barrier ensuring that small food particles do
not escape the mouth apparatus (Garm a).
Many setae have small setules situated on the distal half, as in composite setae, and they
are also often involved in feeding (Schembri , Lavalli and Factor ). They are typically
involved in gentle prey handling (Garm a), and the small and often scalelike setules on the
distal par t probably do not withstand rough handling but will provide a good grip on small par-
ticles . This, and their s lender form, indic ates that they do not ap ply much force to the prey item s.
When not situated on the medial edge of the mouthparts, they are often involved in grooming
the neighboring mouthparts and head region, and their role during gill cleaning is well docu-
mented (Bauer , , ).
Serrate setae with the two rows of denticles on the distal part serve a variety of mechanical
functions. They are found on the mouthparts of almost all examined crustaceans, but again,
their func tion is best studied in decapods. Most are situated on the food-handling medial edges
and per form rough prey h andli ng, such as hold ing and sh redding (Hunt et a l. , Ga rm a).
Another very common mechanical funct ion of serrate setae is grooming of the head region, and
especially the antennae are groomed frequently. In decapods, specialized clusters on the pro-
podus and carpus of the endopod of maxilliped are used in this behavior (Fig. .E–H). Here
Setae, Setules, and Other Ornamentation 185
Fig. 6.7.
Mechanical functions of setae revealed by macro-video recordings. (A) Overview of the mouth appa-
ratus of Astacus astacus feeding on a mussel. (B) Close-up of Pagurus bernhardus eating a piece of fish.
The set ae involved a re found on the me dial ri m of maxi llipe d and (Mxp– ), max illa (Mx ), and th e
mandibular palp (MP). The incisor process of the mandibles (IP) and the labr um (La) are also show n.
(C) Endosco pe recording s can revea l actions of ot herwi se hidden setae . Here large cu spidate seta e from
the bas is of max illa (Mx ) of Panulirus argus are seen b etween the i ncisor process (I P) and media l lobe
(ML). (D) A piece of mussel is held by setae on maxilliped (Mxp) of Penaeus monodon while setae
on the medial rim of maxilliped (Mxp) probe its surface. (E–H) Time series of serrate setae on the
endopod (endo) of maxilliped (Mxp) of P. bernhardus grooming the flagellum of antenna (Ant).
Arrows indicate movements.
186 Functional Morphology and Diversity
the denticles seemingly make the setae very efficient in scraping of debris, and their size can be
correlated with the robustness of the structure they groom (Garm and H ø eg ). Grooming
by serrate setae is well documented in the literature (e.g., Bauer , Pohle , Fleisher et al.
). This suggests that when the serrate setae are found laterally on the mouthparts, such as
the endopods of the maxillae, they may function in grooming the neighboring limbs.
Papposerrate setae combine the long setules on the proximal part with denticles on the
distal part. They have been reported from a number of species (often as plumodenticulate
setae), but their mechanical functions have been studied only in a few species. In the crayfish
Aust ropotamobius pallipes a nd Cherax quadricarinatus , t hey were obser ved to perfor m gentle prey
manipulations such as pushing pieces of prey into the mouth (Thomas , Garm a).
A large number of setae have no outgrowths and appear more or less smooth, and such setae
are found in all crustaceans. Most of them are slender, and their length can vary from a few
micrometers to several millimeters. No mechanical functions have been documented for the
smaller of these simple setae, and the evidence points to them being unimodal chemorecep-
tors (Hipeau-Jacquotte , Elofsson and Hes sler ). The mech anical f unctions of t he larger
simple set ae seem to depend on thei r location. T he aestheta scs on antenna b elong to this g roup,
and they are aga in unimodal chemoreceptors w ith no apparent mechanical funct ions (Hal lberg
et al. ). When situated on the mouthparts of decapods, simple setae have been found to
perform a broad range of mechanical function, which is species dependent (Garm a). In
some species, they appear mainly gustator y with reduced mechanical functions, but in P. argu s ,
they are the dominant setal type performing rough prey handling. In the latter case, the lack of
outgrowths is probably to avoid damage dur ing feeding.
Cu spidate setae are a lso often w ithout outgrow ths, and they seem to per form the most rough
mechanical functions. This is clearly seen on the mouth apparatus, where they are involved in
shredd ing and tear ing of the prey. Stil l, in the herm it crab Pagurus rubricatus , cuspidate set ae on
the medial rim of maxilla can act as a filter (Schembri ). This is a surprising function for
cuspid ate setae and stresses that one should be c areful not to a ssume the fu nctions of setae f rom
their mor phology alone.
Only a few nonsensory functions besides the mechanical functions have been documented
for setae. In a recent study, it was shown from ultrastructural data that some simple setae from
Table 6.2. Summary of the mechanical functions of the seven types of seta.
Seta type Mechanical f unction
Pappose
Setal barriers, current direction, filtering
Plumose
Surface extension of water pumps, setal barriers
Composite
Gentle prey handling (reorientation and relocation of small prey
items), gentle grooming
Serrate
Rough prey handling (collecting and holding prey and shredding soft
prey), rough grooming, f iltering
Papposerrate
Setal barriers, gentle prey handling
Simple
Rough and gentle prey handling
Cuspidate
Very rough prey handling (holding, shredding, and tearing large prey
items), restrict mouthpart movement, f iltering
Setae, Setules, and Other Ornamentation 187
the mouthparts of the remipede Speleonectes tanumekes are associated with glands and might
therefore be involved in secretion (van der Ham and Felgenhauer ). Setae are also bound
to have an i mpact on the hydrodyna mics of cru staceans, which w ill be most important for small
swimming species (see chapter ).
FOSSIL SETAE
The cuticle is advantageous when you want to study the evolutionary history of crustaceans
since it fossil izes easily. Unfortunately, when it comes to fine structu ral detai ls such as the setae,
information is often lacking (e.g., Collins et al. ). A uniquely well-preserved arthropod
fauna, called the “Orsten” fauna (see chapter ), has been found in which the miniature ani-
mals from the Cambrian still display beautiful setae (Fig. .). Surprisingly, these setae display
a great diversity, and several of the seven types we have suggested can be recognized. On the
mouthparts of the fossil crustaceans Rehbachiella , Bredocaris , and members of Skaracarida, at
least cuspidate, composite, and serrate setae were present (Fig. .A–C) (M ü ller and Walossek
, , Walossek ). Plumose and pappose setae were probably also present, but even
though well preserved, most of the setules are broken, impeding certainty about their original
Fig. 6.8.
Fossil setae. (A–C) Orsten fossils of Rehbachiella f rom the Cambr ian show ver y well-preserved setae. T he
mouthparts display many composite and serrate setae. Arrows indicate setules; the arrowhead indicates
denticles. (D) Agnostus pisiformis , also from the Orsten fauna, displays advanced setal types such as plu-
mose setae shown here. Arrows indicate setules. Pictures courtesy of Prof. Dieter Walossek.
188 Functional Morphology and Diversity
length. One important aspect of these findings is that they strongly suggest that at least most of
the setal types we have put forward for crustaceans date back to their very early evolutionary
history. If the setal types represent homologies, this was to be expected since all major recent
subgroups of Crustacea seem to display all the seven types of setae, and setal differentiation
therefore predates their last common ancestor. There is even evidence in the fossil record that
some differentiation happened very early in arthropod history. Another member of the Orsten
fauna, Agnostus pisiformis , which is closely related to trilobites, also displays several of these
advanced seta l types (Fig. .D) (M ü ller and Walossek ).
Only a few authors have addressed the internal relationship and evolution of setae, but it
has been suggested that the most fundamental setal type is the simple seta and that the other
types have evolved from this type by adding more complex features to the setal shaft (Farmer
). If so, one wou ld expect to f ind many sim ple setae in the ea rly fossil record . Interesti ngly,
the animals from the Orsten fauna seem to have very few simple setae (M ü ller and Walossek
, , , Walossek ). The alternative hypothesis is that simple setae have arisen
from other setal types by reduction of the outgrowths. This is supported by setules and den-
ticles being general features of the cuticle and not structures that have evolved exclusively
on the setal shaft. Setules and denticles in the general cuticle appeared early in arthropod
history, which is also seen in the Orsten fauna, where these structures are numerous on the
appendages (Fig. .C) (Walossek ). The general lack of simple setae on the mouthparts
of Orsten animals can perhaps be explained by their small size, since small present-day crus-
taceans in general have few simple setae on their mouth appendages (e.g., H ø eg et al. ,
Olesen ). Further, simple setae have limited f unctional ity, and more complex setae would
be needed to support the likely suspension feeding habit of these small animals. The little
change in the external morphology of setae during the last million years also indicates
that the mechanical functions are preserved.
NONSETAL STRUCTURES OF THE CUTICLE
Compared to setae, much less is known about development and functions of other cuticular struc-
ture s. Setules i n the general c uticle seem to be fou nd throughout Cr ustacea but a lmost exclu sively
around the mouth apparatus. They are never innervated, and thus they have no sensory func-
tions. Their mechanical function has been studied only in a few decapods and only indirectly
(Garm a). They are found on the paragnaths of some decapods, where they fill the space
between the paragnath and max illa . In t his way, they prevent small food par ticles from escaping
and groom the oral surface of maxilla . From their arrangement in other crustaceans, it is likely
that t hey serve sim ilar fu nctions (e.g., Ale xander , Olesen ), and it is noteworthy t hat this
coincides with the functions they serve when situated on setae. Whether setae have adopted set-
ules a long with some of t heir func tions from the ge neral cutic le or whether cut icular set ules repre-
sent heav ily reduced set ae are unk nown. The fo ssil record does not she d any light on t his question
since even Agnostus filiformis had both composite setae and setules on the cuticle (M ü ller and
Walossek ).
Denticles are found in the general cuticle of many crustaceans and can be found on most
body parts. Like setules, they are not sensory, but no other function has been associated with
them. They could play an important role in hydrodynamics, as has been shown for the dermal
teeth of shark s (Lang et a l. ). Stil l, denticles are of ten found on the mouth parts and be tween
fields of setae (Fig. .D), where they will have no contact with prey items or have effect on the
hydrodynamics, and their function here remains enigmatic. Spines, on the other hand, have
well-documented functions and serve mainly in rough prey handling and predator avoidance
Setae, Setules, and Other Ornamentation 189
(Briones-Fourzan et al. ). Spines are mostly found on the appendages, such as antenna of
spiny lobsters, but can be found in many other places.
T here are many ot her cuticu lar str uctures t hat are nonar ticula ted. Klepa l and Kast ner ()
arranged such features into the following categories: hump, protuberance, scale, tooth, fringe,
spine, and comb. Humps and protuberances are round or irregularly shaped areas, respectively,
that push outward from the surrounding cuticle. They have been documented from tanaids
and cumaceans. Scales are flat structures, usually curved, and as wide as or wider than long,
although these dimensions may vary (see Klepal and Kastner , their figs. –). Teeth are
solid, cone-shaped structures of varying length, and when together in a row, they form combs.
Fringes are similar to combs, but the cuticular structures are longer and may be f lexible. The
one feature that K lepal and Kastner () include in their list of nonsensory structures that
may need further investigation is what they call a “spine.” They define a spine as a large, strong,
hollow, cone-shaped structure, and thei r images clearly show an art iculated base. It is li kely that
these structures are, in fact, cuspidate setae.
W hen it comes to the development of t he nonsetal str uctures, both when f irst appearing a nd
duri ng a molt, little is know n about the processes involved . To ou r knowledge, nobody has stud-
ied the development of setules and denticles in the cuticle. Transmission electron microscopic
data from both early and late intermolt indicates that they are not associated with sheath cells
(Fig. .E), suggesting that their development differs from that of denticles and setules on setae
(see earlier section). This does not support the supposed homology between these structures,
but it is not at all impossible that sheath cells are involved during the formation of the new cuti-
cle and then later degenerate or transform.
COMPARISON WITH OTHER ARTHROPOD GROUPS
When observing the cuticle of other arthropods, it immediately becomes obvious that some of
the structures are homologous to what we have described from crustaceans. Again, the most
notable and best understood is setae, which are found in all examined arthropod subphyla. In
the following, we compare crustacean setae with those from insects and spiders, which, espe-
cially in the former, are well understood (for detailed reviews, see Steinbrecht , Keil ,
).
I nsects have setae, or sensilla , as they are mo st often cal led in this g roup, scattered on a ll body
parts, but just as for crustaceans, they are most commonly found on the appendages and in the
head region. When the detailed external morphology is examined, however, insect setae differ
from crustacean setae, probably at least partly due to their terrestrial habitat. First of all, as a
general trend, they have many fewer and smaller outgrowths; long setules would likely collapse
without the support of the water. Instead, insect mechanoreceptors are long and normally slen-
der setae (Keil ). They also have large leaf- or club-shaped mechanosensory setae special-
ized for gravity sensing. The ability to sense gravity directly in this way is again due to the lack
of surroundi ng water. The cut icle of insect chemosensor y setae always seems to have detect able
pores. In the case of olfactory setae, two systems are present, double- and single-walled setae,
both with numerous pores in most of the setal shaft allowing the odors to contact the ODSs
(Steinbrecht ). The gustatory setae of insects are normally bimodal receptors as in crusta-
ceans, and they also display a prominent terminal pore. The olfactory setae have other speciali-
zations not seen in crustaceans because of the physical differences of their habitats. Since they
detect airborne odors, they need an interface to get them in contact with the ODS in the recep-
tor lymph. This is seen as a complex canal system in the cuticle, where the odors are collected
and transpor ted by so-called odor-binding proteins (R ü tzler and Zweibel ).
190 Functional Morphology and Diversity
The ontogeny of insect setae has been studied in detail and is well understood (Keil ).
The pattern of sheath cells (enveloping cells) and their function are much stricter than in crus-
taceans. Normally, three (sometimes two or four) sheath cells are involved in the development
of the setae: the trichogen, tormogen, and thecogen cell. Their functions are similar to what is
known from crustaceans, and they form the cuticle of the seta, protect the sensory cells, and
produce the receptor lymph. They encircle the receptor cells, which are normally fewer than
in crustaceans, although insect olfactory setae can hold up to receptor cells (Hallberg and
Hansson ).
All insect setae are considered to be sensory and have been found to be mechano-, chemo-,
hygro-, thermo-, and carbon dioxide receptors. As for crustaceans, the most common are
chemo- or mec hanoreceptors , but there seems t o be a more strict d ivision, w ith bimod al chemo-
and mechanosens itive setae bei ng less common. This is pa rtly because insects h ave many more
types of olfactory setae, which all seem to be unimodal as in crustacean aesthetascs (Hallberg
and Ha nsson ). In insect s, the tran sduction mecha nisms ar e known for most of t he modali-
ties, not least because of the powerful molecular tools available to study Drosophila and some
other species (e.g., Caldwell and Eberl ). Insects do have scolopedial mechanoreceptors
similar to those of crustaceans, but most insect mechanoreceptors differ in that they have no
scolopale but instead a tubular body distally in the ODS, where both stimulation and trans-
duction takes place (Keil ). Here the mechanosensitive ODS stops at the setal base, where
movements of the seta distort the tubular body. This is again likely a terrestrial adaptation,
since structurally similar mechanoreceptors are found in terrestrial isopods (Crouau ).
From work on insects, it is also known that thermoreceptors are modified mechanoreceptors
(M ü ller et al. ), which is likely also true for hygroreceptors and carbon dioxide receptors
(Stange and Stowe ).
Setae in spiders are at lea st as diverse as in cr ustaceans and insects, and many of them appear
so sim ilar to cr ustace an setae th at it is dif ficult t o tell them apa rt (Figs. ., .). Still , as in inse cts,
most of them h ave few and short outg rowths (Talar ico et al. ), probably because of t heir ter-
restr ial habit at. Unfort unately, rat her littl e experi mental work h as been perfo rmed on their f unc-
tional morphology. They probably have many mechanical functions, and they are all putative
receptors. One of the mechanical functions that has been studied is how setae aid when adher-
ing to surfaces (Niederegger and Stanislav ). As for crustaceans, there seems to be a good
correlation bet ween the detai led appearance of the set ule-like outgrow ths and thei r mechanica l
function. Very little is known about the chemosensors, but some spider mechanosensors, the
trichobothria, are understood in great detail (Barth and H ö ller ). In trichobothria setae
(Fig. .B), which are always unimodal mechanoreceptors, there is a strict correlation between
the structure and arrangement of the setal shaf t and the sensory properties.
On the ult rastruc tural le vel, the exi sting data f rom spider setae i ndicate that t he sensory cel ls
are si milar to b oth insec t and crus tacean sens ory cell s and arra nged in sim ilar way s. The sensor y
cil ia of the chemoreceptors proceed a long way up the setal sha ft, and in put ative gustatory setae
they are enclosed by a dendritic sheath (Talarico et al. ). In olfactory setae, the dendritic
sheath is missing, and the ODSs lay against the setal cuticle and are in contact with the odors
throu gh a pore syste m as in insec ts. The ul trastr ucture of sp ider mechanorece ptors appears ve ry
similar to insect mechanoreceptors of the tubular body type (Talarico et al. ).
WHAT’S NEXT: INTERTAXON AND INTERMETHODOLOGICAL RESEARCH
What we hope to have illustrated in this review is that a fair amount of knowledge is availa-
ble about surface structures of the crustacean cuticle. What is also evident is that this knowl-
edge stems almost entirely from malacostracans and especially from decapod setae. This is
Setae, Setules, and Other Ornamentation 191
unfortunate when the goal is to have a broad understanding of these structures functionally,
ontogenetically, and evolutionarily.
One of the issues we find of great importance is better understanding of how the different
str uctures in the cut icle group together a nd how these groups d iffer from each other. T his should
be studied in an evolutionary context by finding homologies to define the groups. The develop-
mental processes seem to be the most promisi ng place to start. Only a few malacostraca n species
have been exam ined, and a study comparing setal development from a broad range of crustacean
groups is t herefore highly desirable. A study of the development and for mation of the setu les and
Fig. 6.9.
Setae from spiders. (A) Setae associated with the claws of Nicodamus mainae . Arrows indicate setae very
similar to crustacean serrate setae (compare with Fig. .D). (B) Trichobothria from the palp of Mimetus
hesperus . These mechanosensitive setae closely resemble simple setae of crustaceans and are the most
stud ied and best-u nderstood spide r setae. The op en socket (arrow s) makes t hem very f lexibl e, and in most
case they respond to tiny distortions. (C) Setae from the scopula of Copa flavoplumosa with outgrowths
that resemble setules. One type is similar to crustacean plumose setae (arrows; compare with Fig. .B).
(D) A common ty pe of spider seta f rom the abdomen of Pikelinia tam . T he funct ion is unkno wn, and it has
no obvious counterpart in crustaceans. (E) Setae from the metatarsa of a leg of Pseudolampona emmett .
These setae are so sim ilar to ma ny crustacean composite setae that they can ha rdly be dis tinguished from
them (compare with Fig. .C). Pict ures courtesy of Dr Mar t í n J. Ram í re z.
192 Functional Morphology and Diversity
denticles on the general cuticle is also wanted for comparison with the formation of the similar
structu res on the setae. This is v ital to test their possible homology.
Another of the very interesting aspects for crustacean setae is their functional morphology,
which for a large part can be separated into mechanical functions and sensory functions, as we
have described here. We have put forward a classification system based on the external struc-
tures and indicated that this system also follows their mechanical functions. Whether this is
a general rule within crustaceans is too early to decide, since almost all the data come from
large decapods. What is badly needed here are studies combining behavioral observations and
scanning electron microscopy from several distantly related taxa. It is necessary to make the
behavioral observations such that the actions of identifiable setae can be followed during feed-
ing, grooming , walki ng, and so forth. T his is cert ainly a challenge, but with moder n high-speed
and high-resolution video equipment, it is also possible.
The last point we want to make concerns the sensory functions of setae. The bulk of the
available evidence suggests that the most common ty pe of sensory seta in cr ustaceans i s a bimo-
dal che mo- and mechanore ceptor. This nee ds solid conf irmation f rom more setal t ypes throu gh
electrophysiological experiments. By combining such studies with high-quality transmission
elect ron microscopy, the st ructu ral basis for t hese modalit ies can also be fin ally esta blished. It is
also of t he highest interest to get better con firmation on the possible presence of osmo-, hygro-,
and thermosensitive setae. Since there is evidence from insects that all of these modalities are
closely linked to mechanoreception, it is likely that they share many ultrastructural features.
This can turn out to cause serious problems to ultrastructural identification of sensory modal-
ity. The bimodal cells are somewhat of a mystery, and finding out how the combined trans-
duction works, along with how the nervous system separates the information, is of the greatest
interest, not only for crustacean sensor y biology but also for neuroscience in general.
ACKNOWLEDGMENTS
We thank Associate Professor Nikolaj Scharff (Natural History Museum of Copenhagen) and
Mart í n J. Ram í rez and Mat í as A. Izquierdo (Museo Argentino de Ciencias Naturales) for pro-
viding pictures of arachnids (all parts of the Assembling the Tree of Life: Phylogeny of Spiders
project). Further, we are grateful for the pictures provided by Associate Professor Jens H ø eg
(Universit y of Copenhagen) and Professor Dieter Walosz ek (University of U lm) of Parapagurus
sulcata and fossil arthropods, respectively. We also acknowledge the financial support from the
Danish Science Council to A.G. (grant ––).
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